What strategies of motor unit recruitment are u by the CNS to control the musculoskeletal system? The answer to this question is currently incomplete, but significant progress toward this answer will be made by meeting the four specific aims of this proposal. Each aim is to test a principle that may be used in the nervous system's selection of motor units for particular tasks. Specific Aim 1 tests the hypothesis that units are selected for the contribution they make to torque production at a joint. Specific Aim 2 tests the hypothesis that units are selected for their membership in functionally discreet neuromuscular compartments. Specific Aims 3 and 4 test the hypothesis that units are selected for their speeds of contraction and/or relaxation. Experiments planned to meet Aim 4 expand ongoing recruitment investigations in this laboratory to include human subjects. We will explore the as yet unconfirmed claim that slow contracting motor units are selectively inhibited during volitionally controlled lengthening contractions. This strategy is postulated to provide control over the speed of joint rotation during active muscle lengthening. By monitoring single units from a muscle in the hand, we will test the generality of earlier reported findings for ante extensor muscles. The remaining aims will be met using decerebrate cats and applying techniques similar to ones used previously in this laboratory to examine recruitment sequences of physiologically-characterized motor units. Aim 3 is designed to test the effects of stimulating the rubrospinal tract because this tract is predicted to selectively reverse the usual recruitment order. The supraspinal influences are of interest also because the selective recruitment behaviors observed in humans, but not by us thus far in cats, may depend upon these influences. The remaining Aims 1 and 2 will test the extent to which recruitment is organized around geometric relationships between motor units.and the skeletal system. This sort of- organization would be revealed if recruitment is found to be related to motor unit torque or motor unit membership in muscle compartments that produce different torques. The long term objective is to define neural strategies applied in producing normal movements. We believe that this information would be extremely valuable to rehabilitation medicine; it would assist in diagnosing motor problems and in assessing the effectiveness of various treatments, and would also provide a foundation for developing treatments to restore normal movement after debilitating injuries such as spinal cord trauma.